I don't think that Sandra Tanner should be excluded simply because of bias. If she cites peer-reviewed sources for her claims, which can be researched for confirmation, then her offerings should be accepted regardless of the messenger's bias. The problem is that Nibley cites peer-reviewed sources for most of his claims, and from what I've seen Sandra Tanner and her husband either don't cite, cite things that don't need to be cited, or just cite other anti-Mormon works.
Bathsheba has unintentionally identified the problem here, by talking about using Nibley as a "source" or a "resource." In the parlance of debate, Nibley is the latter but not the former. He's a messenger, but the message is from Knudson and all the other non-Mormon sources that he cites. When someone asks me for my "source data," I'm supposed to cite Knudson, not Nibley. the term for Nibley is an "intermediary source" or just a "resource."
There are plenty of LDS scholars who I refuse to use as sources, not because they are biased (we all are) but because they do a bad job of citing their sources. Milton R. Hunter, for example, is not half so good as Nibley, and for that reason I usually flip through his footnotes and go straight to the sources.
But there's the rub: in the end, I don't think it's fair to critics of the church to exclude Tanner and Nibley as "sources," because I can continue to use Nibley as a "resource" without citing him. I would just go on Lexus-Nexus and look up all of Nibley's sources and cite them individually. But if Sandra Tanner has actually done her homework, I'd like her to get credit, too. Bring it on, Sandy!